The present invention relates to making semiconductor components and more particularly relates to devices for growing epitaxial layers on substrates, such as wafers.
Various industries employ processes to form thin layers on solid substrates. The substrates having deposited thin layers are routinely used in microprocessors, electro-optical devices, communication devices and others. The processes for deposition of thin layers on solid substrates are especially important for the semiconductor industry. In the manufacturing of semiconductors, the coated solid substrates, such as substantially planar wafers made of silicon and silicon carbide, are used to produce semiconductor devices. After the thin firm is deposited, the coated wafers are subjected to well-known further processes to form semiconductor devices such as lasers, transistors, light-emitting diodes, and a variety of other devices. For example, the thin layers deposited on the wafer form the active elements of the light-emitting diodes.
The materials deposited on solid substrates include silicon carbide, gallium arsenide, complex metal oxides and many others. The thin films of inorganic materials are typically deposited by processes collectively known as chemical vapor deposition (CVD). It is known that the CVD processes, if properly controlled, produce thin films having organized crystal lattices. Especially important are the deposited thin films having the same crystal lattice structures as the underlying solid substrates. The layers by which such thin films grow are called the epitaxial layers.
In a typical chemical vapor deposition process, the substrate, usually a wafer, is exposed to gases inside a reaction chamber of a CVD reactor. Reactant chemicals carried by the gases are introduced over the wafer in controlled quantities and at controlled rates, while the wafer is heated and usually rotated. The reactant chemicals, commonly referred to as precursors, may be introduced into the CVD reactor by placing the reactant chemicals in a device known as a bubbler and then passing a carrier gas through the bubbler. The carrier gas picks up the molecules of the precursors to provide a reactant gas that is then fed into the reaction chamber of the CVD reactor. The precursors typically include inorganic components, which later form the epitaxial layers on the surface of the wafer (e.g., Si, Y, Nb, etc.), and organic components. Usually, the organic components are used to allow the volatilization of the precursors in the bubbler. While the inorganic components are stable to high temperatures inside the reaction chamber, the organic components readily decompose upon heating to a sufficiently high temperature.
When the reactant gas reaches the vicinity of a heated wafer, the organic components decompose, depositing the inorganic components on the surface of the wafer in the form of the epitaxial layers. If the wafer does not have a sufficiently high temperature, the extent and the rate of the decomposition reaction, and therefore the deposition, may be lower than necessary to ensure efficient deposition and growth of the epitaxial layers. Further, depending on the nature of the inorganic components and the reactant gas, different temperature requirements exist for different types of CVD processes. For example, it is known to one skilled in the art that the deposition of silicon carbide (SiC) may require wafer temperatures of up to 1600xc2x0 C. or higher, while the deposition of other typical semiconductor films, such as transition metal oxides, may efficiently proceed at 600xc2x0 C. to 800xc2x0 C. Therefore, the requirements for heating methods and equipment may be rather demanding and may vary as a function of the specific CVD application.
Among the requirements for any heating methodology used in the CVD processes are heating uniformity, high heating rate, ease of temperature control and high temperature tolerance for component parts. Additional considerations, such as prices of the required component parts, ease of maintenance, energy efficiency and minimization of the heating assembly""s thermal inertia, may be equally important. For example, if the heated components of a CVD reactor have high thermal inertia, certain reactor operations may be delayed until the heated components reach the desired temperatures. Therefore, lower thermal inertia of the heated components of the reactor increases the productivity since the throughput depends upon the length of the reactor cycle.
At present, two major heating methodologies are used in the CVD reactors: radio frequency (RF) heating and radiant heating. A typical CVD reactor with RF heating is disclosed in U.S. Pat. No. 5,186,756, and includes radio frequency coils (RF coils), typically located outside the reaction chamber. The radio frequency emitted by the RF coils is used to heat a wafer inside the reaction chamber while the wafer is held on a susceptor, which is a wafer-supporting element mounted in the reaction chamber. The typical susceptor suitable for the RF-heated CVD reactor is made from a highly temperature resistant and usually very expensive material, such as molybdenum.
RF heating permits a very high rate of heating, which is advantageous. Also, the RF coils in general have a long reactor lifetime, which is also desirable. However, at the same time, RF heating has a number of significant drawbacks and, for this reason, is less common in the modem CVD reactors than radiant heating. Among the drawbacks are high prices of the component parts, difficulties in maintenance, high thermal inertia of the heating assembly, the necessity for a specially trained work force associated with the utilization of high frequency output devices, the high level of potential health hazard and the large size of the heating assembly.
In general, the CVD reactors with radiant heating have several important advantages over the RF-heated reactors. Importantly, such CVD reactors have a smaller and less expensive heating assembly and lower maintenance/training requirements for the manufacturing personnel. Usually, the CVD reactors with radiant heating utilize one or more radiant heating elements located inside the reaction chamber in proximity to a wafer-supporting assembly. The radiant heating elements typically include heating filaments made of graphite or other similar material and are less expensive than complex RF heating coils. Very importantly, use of localized radiant heating instead of less discriminating radio frequency heating permits selective heating of various parts of the wafer-supporting assembly by separate radiant heating elements. Such selective localized heating, which is commonly referred to as the multi-zone heating, provides excellent control over heating uniformity that is highly desirable in the CVD processes. Also, the graphite construction of the heating filaments provides low thermal inertia for the heating filaments and good filament-to-filament reproducibility. All of these factors have resulted in the preferential use of radiant heating in the semiconductor industry.
However, while RF heating is on the decline in most CVD applications, it is still common in the CVD reactors used for the deposition of silicon carbide (SiC). As discussed above, the deposition of SiC requires rather high wafer temperatures, often in excess of 1600xc2x0 C. The high heating rates and the thermal stability of the components of RF heating and/or wafer-supporting assemblies have allowed the CVD reactors with RF heating to maintain their presence in the commercial marketplace despite the widespread prevalence of radiant heating in other CVD applications. In addition, the presently available CVD reactors with radiant heating have a number of significant limitations with respect to their use for the deposition of SiC on wafers. These limitations will be discussed further with reference to the existing prior art CVD reactors with radiant heating.
CVD reactors with radiant heating have various designs, including horizontal reactors in which wafers are mounted at an angle to the inflowing reactant gases; horizontal reactors with planetary rotation in which the reactant gases pass across the wafers; barrel reactors; and vertical reactors in which the wafers are rotated at a relatively high speed within the reaction chamber as reactant gases are injected downwardly onto the wafers. The vertical reactors with high-speed rotation are among the most commercially important CVD reactors.
A typical vertical, prior art CVD reactor with radiant heating is illustrated in FIG. 1. As seen with reference to FIG. 1, a wafer 10 is placed on a wafer carrier 12, which is placed on a susceptor 14. The susceptor 14 is usually made from an expensive, highly thermally-stable material, capable of withstanding a large number of reactor cycles, such as molybdenum. On the other hand, the wafer carrier 12 is made from a material that is relatively less expensive and allows good manufacturing reproducibility since the wafer carrier is typically replaced after a certain commercially suitable number of reactor cycles.
The susceptor 14 is permanently mounted on a rotatable spindle 16, which enables the rotation of the susceptor 14, the wafer carrier 12 and the wafer 10. The susceptor 14, the wafer carrier 12 and the wafer 10 are located in an enclosed reactor chamber 18. A radiant heating element or assembly 20, which includes one or more heating filaments 22, is arranged below the susceptor 14, and is heated by passing electric current through electrodes 25. The heating assembly 20 heats the susceptor 14, the wafer carrier 12 and, ultimately, the wafer 10. During the deposition, the wafer-supporting assembly (spindle/susceptor/wafer carrier) is rotated while the reactant gas is introduced into the reaction chamber 18 over the heated wafer 10, depositing a film on the surface of the wafer. The rotation of the wafer 10 is intended to enhance the temperature uniformity across the deposition area, as well as the uniformity of the reactant gas introduced over the wafer 10 during the deposition.
The prior art apparatus, with radiant heating similar to or different from the apparatus shown in FIG. 1, enjoy a widespread and successful use for a variety of CVD applications. For example, Daud E series vertical high-speed rotating reactor, made by Emcore Corporation of Somerset, N.J., is one of the most successful CVD reactors in the commercial marketplace.
Nevertheless, the need exists for a CVD reactor that utilizes radiant heating for the deposition of silicon carbide. Further, the need also exists for a relatively inexpensive and reliable CVD reactor for a variety of CVD applications that has low thermal inertia and extended filament lifetime.
The present invention addresses these needs by providing a novel CVD reactor in which wafers are placed directly on the surface of the heating filament, a novel method of growing epitaxial layers in a CVD reactor and an assemblage for use therewith.
The inventors have identified a number of obstacles and limitations of the presently available prior art vertical CVD reactors that may affect their use in silicon carbide (SiC) deposition. However, it should be understood that the apparatus of the invention may be used for a variety of CVD applications.
Thus, it has been determined by the inventors that substantial thermal losses occur at the thermal interfaces in the wafer-supporting/heating assembly of the existing CVD reactors. The CVD reactor having both a susceptor and a wafer carrier, such as the reactor shown in FIG. 1, contains at least two of such interfaces. Referring to FIG. 1, these are the interfaces between the radiant heating element 20 and the susceptor 14, and between the susceptor 14 and the wafer carrier 12. Research by the inventors has shown that a substantial temperature gradient is present at these interfaces, e.g., the temperature of the radiant heating assembly 20 is substantially higher than the temperature of the susceptor 14, which, in turn, is higher than the temperature of the wafer carrier 12. Consequently, the radiant heating element 20 must be heated to a substantially higher temperature than the temperature desired for the wafer 10 during the deposition. The required higher temperatures of the heating assembly lead to higher energy consumption and faster deterioration of the heating filaments 22.
For example, the research by the inventors has shown that in a CVD reactor similar to the reactor of FIG. 1, and operating at wafer temperatures of over 1600xc2x0 C., the temperature gradient at each interface is approximately 250-300xc2x0 C. Therefore, for example, to produce the temperature of 1600xc2x0 C. for the wafer 10, which is typically required for SiC deposition, the temperature of the heating filaments 22 may have to reach 2200xc2x0 C. While a graphite filament may have a commercially acceptable lifetime at 1600xc2x0 C., the inventors have determined that the lifetime of the heating filament rapidly decreases and becomes commercially unfeasible as the temperatures of the filament approach and exceed 2000xc2x0 C. Thus, for example, at 2200xc2x0 C., the heating filaments have a very short and commercially unsuitable lifetime, exhibiting a significant reduction in the cross-sectional area of the filaments and the consequent filament deterioration. This is a principal limitation on the use of radiant heating in the SiC deposition.
Separately, a typical susceptor must withstand a large number of reactor cycles and, therefore, the susceptors are usually made from highly temperature- and deformation-resistant materials, such as molybdenum. Such materials are very heavy and expensive, and often make a significant contribution to thermal and mechanical inertia of the wafer-supporting assembly. The increased thermal inertia leads to a substantial increase in the time required to heat and cool down the wafer-supporting assembly, resulting in a longer reactor cycle and consequent reduction in the productivity of the reactor. The increased mechanical inertia increases the strain on the spindle-supporting assembly, thereby reducing its lifetime.
To overcome these obstacles and limitations of the presently available CVD reactors, the present invention provides a novel CVD apparatus in which the filament also functions as a susceptor, i.e., the wafers are placed directly on the surface of the heating filament. In one embodiment of the apparatus of the invention, the filament is permanently mounted at the location where chemical vapor deposition takes place. In another embodiment, the filament also serves the function of a wafer carrier, i.e., the filament may be transported away from the location where the deposition takes place to load and unload the wafers. In both embodiments of the CVD apparatus of the invention, the filament/susceptor and the susceptor/wafer carrier interfaces are eliminated. The elimination of the filament/susceptor and susceptor/wafer carrier interfaces allows a significant reduction in the required filament temperature, and leads to lower thermal and mechanical inertia of the wafer-supporting assembly.
The inventors"" research has also shown that, at lower temperatures of the heating filament of the apparatus of the invention, the lifetime of the filament is significantly extended. For example, the filament lifetime may exceed 100 hours at filament temperatures of up to 1800xc2x0 C., which is well within the commercially acceptable filament lifetimes.
Therefore, according to one aspect of the invention, an apparatus for growing epitaxial layers on wafers is provided in which the wafers are placed directly on the surface of a heating filament, which is both heating and providing a support for the wafers during the chemical wafer deposition process.
According to one embodiment of the apparatus of the invention, the apparatus includes a reaction chamber, a rotatable spindle, a plurality of rotatable electrodes mounted on the spindle for rotation together with the spindle, and a heating filament in electrical contact with the rotatable electrodes. The heating filament may be rotated by rotating the rotatable electrodes, and heated by providing electric current to the electrodes. In this embodiment, both the heating filament and the rotatable electrodes are located inside the reaction chamber. Preferably, the heating filament is permanently mounted on the plurality of rotatable electrodes. In this embodiment, the wafers are placed on the surface of the heating filament by transferring them into the reaction chamber.
According to another embodiment of the apparatus of the invention, the apparatus includes a reaction chamber, a rotatable spindle, a plurality of rotatable electrodes mounted on the spindle for rotation together with the spindle, and a heating filament detachably mounted on the rotatable electrodes. In this embodiment, the heating filament is not permanently mounted on the plurality of the rotatable electrodes. Instead, the filament may be detached from the rotatable electrodes to load or unload the wafers. When mounted on the rotatable electrodes, the heating filament is in electrical contact with the electrodes thereby the filament may be heated by passing electric current through the electrodes. Preferably, the heating filament is transported between a deposition position and a loading position. In the deposition position, the heating filament is detachably mounted on the plurality of rotatable electrodes for rotation together with the electrodes. The loading position is the position in which the filament is detached from the plurality of rotatable electrodes and to which it is transported to load or unload the wafers.
The apparatus may also include mechanical means for transporting the heating filament between the deposition position and the loading position. Also, according to this embodiment of the invention, the apparatus may include a separate retaining means for retaining the filament in the deposition position, for example, to prevent dismounting of the filament while it is rotated, and to improve the electrical contact between the heating filament and the plurality of the rotatable electrodes.
Both embodiments of the apparatus of the invention may share common features. Preferably, in both embodiments of the invention, the heating filament is directly mounted on the plurality of rotatable electrodes. The terms xe2x80x9cdirect contactxe2x80x9d and/or xe2x80x9cdirectly mountedxe2x80x9d are defined to mean a direct physical contact between the identified elements of the apparatus.
Further describing the preferred general features according to both embodiments of the apparatus of the invention, the heating filament may include a top surface having one or more cavities for retaining the wafers, a bottom surface and a perimeter. Thus, according to the present invention, the apparatus may be utilized for coating either a single wafer or plurality of wafers at the same time. The rotatable spindle of the apparatus of the invention may have an axis of rotation, an upper end disposed inside the reaction chamber, a lower end disposed outside the reaction chamber and an inside opening extending between the upper end and the lower end of the spindle. The reaction chamber of the apparatus of the invention may include a horizontal base plate defining a spindle opening for inserting the spindle. The spindle may be inserted through the spindle opening of the horizontal base plate in such a manner that the axis of rotation of spindle is substantially perpendicular to the horizontal base plate of the reaction chamber. At least two of the plurality of rotatable electrodes are spaced apart from each other, defining a gap. Preferably, the heating filament is mounted on the rotatable electrodes to bridge the gap between the rotatable electrodes.
Yet further describing the preferred general features according to both embodiments of the apparatus of the invention, the apparatus of the invention may also include a vacuum rotating feedthrough in sealing engagement with the spindle for providing a vacuum seal between the spindle and the reaction chamber. The apparatus of the invention may also include a motor connected to the rotatable spindle for selectively rotating the spindle and the plurality of rotatable electrodes mounted on the spindle.
To provide the electric current to the rotatable electrodes located within the reaction chamber, the apparatus of the invention may also include a plurality of stationary electrodes located outside the reaction chamber and means for transmitting the electric current from the stationary electrodes to the rotatable electrodes. This means of transmission may include a plurality of connecting electrodes sealed inside the spindle and extending between the rotatable electrodes and the stationary electrodes through the inside opening in the spindle.
Preferably, the heating filament is manufactured from a material capable of withstanding temperatures of up to at least 1600xc2x0 C., enabling the use of the heating filament for the deposition of silicon carbide (SiC) on wafers. Most preferably, this material is graphite.
According to both embodiments of the apparatus of the invention, the apparatus may also include a separate retaining means for retaining the heating filament on the plurality of rotatable electrodes. The choice of the suitable retaining means may vary for each embodiment of the apparatus of the invention, as discussed further with reference to the detailed description of the invention. Also, it should be noted that each of the rotatable electrodes may be either constructed as a single whole, i.e., have unitary construction, or may be constructed from separate parts, without deviating from the invention described herein.
In the preferred embodiment of the apparatus of the invention, the heating filament is detachably mounted on the plurality of rotatable electrodes. In this embodiment, the heating filament is transported between the deposition position and the loading position via mechanical means. Thus, the apparatus of this preferred embodiment of the invention may also include the mechanical means for transporting the heating filament from the position detachably mounted on the plurality of rotatable electrodes (deposition position) to the position for loading or unloading the wafer (loading position).
According to this preferred embodiment of the apparatus of the invention, the reaction chamber of the apparatus of the invention may include a horizontal base plate defining a spindle opening for inserting the spindle. The spindle, which has an axis of rotation and an upper end, may be inserted through the spindle opening in the horizontal base plate in such a manner that the axis of rotation of spindle is substantially perpendicular to the horizontal base plate of the reaction chamber, thereby the upper end of the spindle is located inside the reaction chamber. In this embodiment, the heating filament may include a top surface having one or more cavities for retaining the wafers, a bottom surface and a perimeter.
While the invention is further described with reference to the specific features of various variants of the invention, it should be understood that these features may be included in other variants of the invention.
In one variant of the preferred embodiment of the invention, each of the plurality of rotatable electrodes includes a horizontal portion and a vertical portion, with the horizontal portions extending radially outwards from the axis of rotation of the spindle. Each horizontal portion includes a near end located proximal to the axis of rotation of the spindle, and a far end located distal from the axis of rotation of the spindle, with the vertical portions extending vertically upwards from the horizontal portions. Each horizontal portion is terminating in an electrode end point, with all of the electrode end points lying at a substantially the same elevation. Preferably, the vertical portions extend from the far ends of the horizontal portions.
Separately describing the heating filament and the rotatable electrodes of this variant of the invention, the bottom surface of the heating filament includes a plurality of recesses extending upwards from the bottom surface of the heating filament. Preferably, the recesses do not reach the top surface of the heating filament. Thus, each recess is terminating in a recess end point located at a lower elevation that the top surface of the filament.
The heating filament, which preferably has a substantially round shape, includes thicker portions and a thinner portion. The thinner portion separates the thicker portions of the heating filament, which include the recesses in the bottom surface of the filament. Preferably, the thicker portions are located adjacent to the perimeter of the heating filament. The thinner portion includes a wafer region lying between the thicker portions of the heating filament. The wafer region contains the cavities for retaining the wafers on the top surface of the filament. The number and the locations of the recesses extending from the bottom surface of the filament through the thicker portions of the filament matches the number and the locations of the electrode end points of the rotatable electrodes described above.
Now describing the relationship between the heating filament and the rotatable electrodes in the deposition position, the electrode end points of the rotatable electrodes are inserted in the matching recesses in the bottom surface of the heating filament, thereby mounting the filament in the deposition position. Most preferably, in this variant, there are four rotatable electrodes.
In another variant of the preferred embodiment of the invention, each electrode of the plurality of rotatable electrodes includes a horizontal plate and a vertical plate, with the horizontal plates having a far portion and a near portion. To show the spatial relationship between the horizontal and vertical plates, they are described with reference to an imaginary vertical plane that is substantially perpendicular to the base plate of said reaction chamber. This imaginary vertical plane may or may not include the axis of rotation of the spindle, i.e., the axis of rotation of the spindle may or may not be co-extensive with the imaginary vertical plane. From this frame of reference, in this variant, the horizontal plates of the rotatable electrodes are extending away from the imaginary vertical plane whereby the near portions of the horizontal plates are proximal to each other and the far portions of the horizontal plates are distal from each other. The vertical plates are extending vertically upwards from the far portions of each of the horizontal plates, terminating in electrode end points.
Further describing this variant of the apparatus of the invention, each of the plurality of rotatable electrodes may also include a roof member attached to the electrode end points of each vertical plate and extending toward the reference imaginary vertical plane. Each of the vertical plates of the rotatable electrodes includes an internal surface having a horizontal slit located near the corresponding electrode end point. The horizontal slits lie at substantially identical elevations, which are lower than the elevation of the electrode end points.
Now describing the relationship between the heating filament and the rotatable electrodes in the deposition position, the heating filament preferably has a substantially rectangular shape, with the top surface and the bottom surface of the filament being substantially parallel to each other. To mount the heating filament in the deposition position, the parts of the perimeter of the filament are inserted into the above-described horizontal slits in the internal surfaces of the vertical plates of the rotatable electrodes, whereby the heating filament is in electrical contact with the rotatable electrodes.
The apparatus of the invention may also include a separate clamping means for clamping the heating filament in the deposition position. The clamping means may be used to improve the electrical contact between the heating filament and the rotatable electrodes. Preferably, with reference to the heating filament being in the deposition position, the clamping means are disposed between the heating filament and the horizontal plates of the rotatable electrodes. To provide the clamping action, the clamping means may include a springing means for exerting a clamping force in a vertically upwards direction thereby pressing the heating filament against the roof members attached to the electrode end points, whereby improving the electrical contact between the heating filament and the rotatable electrodes. Preferably, in this variant of the invention, there are two rotatable electrodes.
According to another aspect of the invention, a method of growing epitaxial layers on wafers is provided, including
(a) providing a deposition apparatus that includes a reaction chamber,
(b) providing a heating filament having a surface for placing the wafers within the reaction chamber,
(c) placing the wafers directly on the surface of the heating filament; and
(d) rotating the heating filament within the reaction chamber while supplying an electric current to the heating filament.
According to the method of the invention, the step of supplying the electric current to the heating filament preferably includes directly mounting the heating filament on a plurality of rotatable electrodes. The method of the invention may also include introducing one or more reactants into the reaction chamber.
Most preferably, the step of supplying the electric current to the heating filament includes detachably mounting the heating filament on a plurality of rotatable electrodes. Thus, the step of placing the wafers on the surface of the filament may include detaching the heating filament from the plurality of rotatable electrodes, transporting the heating filament outside the reaction chamber to a loading position, loading the wafers to be deposited onto the heating filament, and transporting the heating filament with the wafers onto the plurality of rotatable electrodes. The method of the invention may be used for the deposition on a single wafer or a plurality of wafers.
According to yet another aspect of the invention, an assemblage for supporting and heating one or more wafers in an apparatus for chemical vapor deposition is provided. The assemblage includes a heating filament having a surface for directly placing the wafer and a plurality of rotatable electrodes for mounting the heating filament and providing electric current to the filament to heat the filament. In the assemblage of the invention, the filament is directly and detachably mounted on the plurality of rotatable electrodes. In one of the preferred embodiments, the assemblage further includes a separate retaining means for retaining the filament on the plurality of rotatable electrodes while the electrodes are rotating.
Additional features and advantages of the invention will be set forth in the detailed description of the invention which follows. It should be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.